nova thread tools

nova thread tools aims to provide cross-platform implementations of
thread synchronization functions.

provided classes:
- nova::dummy_mutex: dummy class, implementing the TryMutex concept
- nova::spin_lock: spin lock (pthread wrapper)
- nova::nonrecursive_rw_mutex: non-recursive reader-writer mutex
  (pthread wrapper)
- nova::rw_mutex: recursive reader-writer mutex (pthread wrapper)
- nova::rw_spinlock: nonrecursive reader-writer spinlock
- nova::semaphore: semaphore class (posix/boost wrapper)

provided functions:
- nova::nanosleep: sleeping functions with nano-second resolution (if
  supported by operating system)
- nova::thread_priority: get priority of current thread
- nova::thread_priority_interval(_rt): get valid priority interval
- nova::thread_set_priority(_rt): set priority of current thread


dependencies:
- boost.thread
- boost.lockfree (for memory barriers and atomic operations)